1. Field of the Invention
The present invention generally relates to the design, manufacture and operation of cables for delivery of electrical power and, more particularly, to power cables for special applications where power cables must be operated over a wide variety of environmental conditions or at or near actual power delivery capacity.
2. Description of the Prior Art
The variety of environments and circumstances where delivery of electrical power over a significant distance through a cable is required has expanded virtually without limit, particularly in recent years. Many such applications present relatively stable operating conditions and common characteristics of cable installation such as power distribution wiring in buildings, power lines, manufacturing facilities (where the environmental conditions may be particularly harsh but are largely consistent over time), appliances, computers (including large installations), instrumentation, hospitals (which may include installations of apparatus with high and intermittent power consumption), ships, aircraft and the like. In general, where the demand is adequate to support volume production and sales, special types, designs and/or grades of wiring have been developed to satisfy most applications at the present time. Such cable designs often incorporate built-in design margins for anticipated operating or environmental conditions such as specification of a minimum wire gauge for a given current-carrying requirement, particular insulation materials for use in the vicinity of heat sources and/or high temperatures or particular structural properties to resist attack by ambient materials or damage from mechanical forces.
The current carrying capacity (sometimes referred to as “ampacity”) of electrical power cables is generally limited by allowable internal temperature rise due to heat generated by ohmic losses (sometimes referred to as “copper losses” regardless of the material from which the cable is made or, infrequently, “aluminum losses” or the like) due to electrical resistance in the cable and the ability of insulation to withstand such internal temperature rise. In addition to the bulk electrical resistance of the cable conductor material, electrical resistance may also be altered by temperature, static and dynamic forces applied to the cable and other mechanical effects (e.g. distortion, metal fatigue and the like) which may locally or globally damage sections of the conductor. Plastic insulation, in particular, which is in direct contact with a hot conductor may be partially or fully melted or may exhibit early loss of dielectric properties at high temperatures potentially causing internal short-circuiting and cable failure. Built-in design margins referred to above are largely directed to guaranteeing that temperatures at which cable degradation can occur are not reached or even approached over a wide range of installation conditions (including foreseeable damage incident thereto) and operating environments for the current loads for which the cable is designed. When standard cable designs having such built-in design margins cannot be used, particularly in high-performance applications such as where size or weight presents severe design constraints or so-called reeled applications where the cable is used while wound on a spool presenting many layers that retain and concentrate generated heat, predicting the temperature rise for all possible conditions can be difficult and can lead to conservative design and unused capacity with the corresponding costs and inefficiencies.
However, many more applications exist where the operating environment is much more variable and the cable design more critical and where built-in design margins cannot be observed. It should be recognized that built-in design margins necessarily carry a cost and inherent inefficiency because the built-in design margins provide additional power-carrying capacity which is not used. Conversely, optimization of a power cable can result in major overall savings in cable cost and size and cost of equipment for handling the cable. For example, in machinery used for marine, underwater applications, optimization of cable size and weight may be reflected not only in the machinery for directly handling the cable but throughout most of the vessel, including the ultimate size thereof. On the other hand, optimization of cable design essentially is an exercise in lowering design margins in regard to temperature, mechanical stresses including flexure, damage or the like and general cable operating conditions as much as possible, thus, in such circumstances as in this example, increasing the risk of failure offshore where the costs attributable to a failure can be enormous. At the present state of the art, one way to limit the risk of such failure has been to design additional current-carrying capacity and other protective structures into the cable even though the cost and inefficiency of doing so can also be quite large. For many applications, the costs of providing excess but unused capacity or internal structures in electrical power delivery cables may significantly increase their size and weight or compromise other desirable properties such as flexibility and the risk and attendant costs of cable failure must be carefully managed.
Embedded fiber optic sensors are sometimes used to monitor cable temperature but such sensors are proprietary and subject to errors and other physical effects such as cable strain. Determining cable temperature by a measurement of wire resistance is not practical since delivery of power must be interrupted to make the measurement and only provides an average temperature rise measurement for the entire cable while a common failure mode involves localized overheating in a possibly relatively short portion of a very long cable.